Tungsten-rhenium coated ceramic reactor fuel particles



nited States Patent 3,396,080 Patented Aug. 6, 1968 3,396,080TUNGSTEN-RHENIUM COATED CERAlVHC REACTOR FUEL PARTICLES Charles E.Hamrin, Jr., Oak Ridge, Tenn., assignor to the United States of Americaas represented by the United States Atomic Energy Commission No Drawing.Original application Oct. 22, 1965, Ser. No. 502,690, now Patent No.3,343,979, dated Sept. 26, 1967. Divided and this application Apr. 19,1967, Ser. No. 656,965

2 Claims. (Cl. 176-67) ABSTRACT OF THE DISCLOSURE Spherical ceramicnuclear reactor fuel particles are provided with a uniform bondedcoating of tungsten-rhenium metal alloy.

The present invention relates to thermo-chemical reduc tion processesand more particularly to a novel method for co-depositing atungsten-rhenium alloy by such techniques and to articles thereof.

This application is a division of my prior application, Ser. No.502,690, filed Oct. 22, 1965 now Patent No. 3,343,979.

Heretofore, numerous metals have been deposited on supports orrn'andrels by well known techniques of vapor phase deposition.Generally, such methods consisted of thermally decomposing suitablecompounds of the selected metal, which are usually in the form of avolatile metal halide, onto a heated object. Hydrogen reduction of avolatile metal halide has also been used to deposit metals. While suchtechniques have found widespread usage in preparing metals in differentshapes and sizes, these techniques have generally, been inefiectual whenapplied to co-deposition processes. Such difliculty stems, mainly, fromthe different rates of deposition and the fact that at temperatures atwhich one metal deposits at high efficiency other metals which would beof interest as allowing agents deposit at very low efliciencies.Moreover, these problems are compounded where, as for example in thefield of preparing refractory metal coated nuclear fuel particles, thecoating must not only have uniform consistency but also must bedeposited so as to form uniform coatings.

An object is to prepare reactor fuel particles having a uniform coatingof tungsten-rhenium alloy of controlled composition.

A further object is to prepare dense spherical uranium dioxide particlescoated with a 21% Re=79% W refractory metal alloy, said coating beingcharacterized by its uniform thickness and homogenity of constituents.

In accordance with this invention, applicant has deposited atungsten-rhenium metal alloy of controlled composition consisting, forexample, of 21% rhenium-79% tungsten onto spherical uranium dioxideparticles. Coatings deposited in accordance with the hereinafterdescribed process parameters were found to be of uniform homogenity ofconstituents, essentially dense 99% of theoretical density), and ofuniform coating thickness. Leach studies (resulting in losses less thanabout 3%!) indicated only a small portion of the particles had inferiorcoatings.

It has been found that the article of the present invention may ideallybe prepared in a fluidized bed reactor. The design and operation offluidized bed reactors which may be employed in carrying out thisprocess are well known to those skilled in the art and need not beexplained in greater detail than that given herein to provide anadequate explanation of the present invention.

In carrying out the invention spherical uranium dioxide particles arecharged into a fluidized bed reactor. It should be apparent that theprior history, i.e., particular fabricational method, of the substrateforms no part of this invention and may be prepared by any conventionaltechnique. Those skilled in the art will appreciate that while the sizerange of particles employed in fluidized bed coating operations may varyover a wide range, spherical particles having an average particle sizeof between 210 and 500 microns are especially suited for such coatingoperations.

As an initial step of the process, the reactor bed is first purged bypassing an inert gas, such as argon, through the bed particles andduring this purging phase the reactor and its charge are heated to about450 C. While in a fluidized state, hydrogen gas is then passed upwardthrough the charge.

To initiate the coating operation a gaseous reaction mixture of tungstenhexafluoride and rhenium hexafluoride is passed simultaneously into thereactor through a common inlet in the bottom of the reactor vessel andisolated from hydrogen prior to contacting the particles. For thisseparate metered flow rates of tungsten hexafluoride and rheniumhexafluoride are admitted simultaneously through the common inlet. Sincerhenium hexafluoride is a liquid at room temperature (boiling point 33.8C.) and undergoes reaction with moisture or oxygen, transfer of therhenium hexafluoride gas from a source supply to the mixing point priorto entry into the fluidized bed presents handling problems. These may beovercome by bubbling helium through a container containing liquidrhenium hexafluoride which is maintained at a controlled temperaturesuch as 25 C.

It has been found that in the present co-deposition process astoichiometric ratio of rhenium and tungsten (such as 1-3 where forexample a 25% Re=% W alloy is desired) cannot be employed to obtain acorresponding ratio of rhenium and tungsten in the alloy deposited. Thisis generally believed attributable to the fact that rheniumhexafluoride, at a given temperature, undergoes more complete conversionby hydrogen reduction than does tungsten hexafluoride. Deposition flowrates (wF zReF of about 1 to 7 have been suitable and are preferred.With respect to the gaseous reaction mixture, applicant has found that astoichiometric excess of hydrogen should be employed as a diluent.

The temperature at which the co-deposition process is conducted iscritical. Deposition rates of rhenium and tungsten metal are controlledby the temperature within the fluidized bed and have been found, wheredeposited separately, to generally increase as the depositiontemperature increases above about 400 C. Deposition rates were found tobe quite low below about 400 C. and unsatisfactory. While it wouldappear that the present invention should be carried out at highertemperatures due to the increased deposition rates, applicants havefound that vapor deposited rhenium metal when laid down at the higherrates, i.e., at higher temperatures, is of a nodular character andundesirable because of nonuniformity of the coating thickness andinclusions of voids in the coating. On the other hand, while depositionrates of the tungsten hexafluoride favor higher temperatures, tungstenmetal deposited at temperatures between 400 and 550 C. has been found tobe of high quality. Accordingly, temperatures between 400-500 C. arerequired for providing a uniform co-deposition from rhenium hexafluorideand tungsten hexafluoride.

The coating time is not cirtical. It should be apparent that as thecoating time increases the coating thickness increases, and it wouldnaturally follow that the coating time would be varied depending uponthe coating thickness desired. For example, spheroids having a coatingthickness of 29 microns have been prepared in 2 hours at 450 C.

Further illustration of the quantitative aspects and pro cedures of thepresent invention is provided in the following example:

EXAMPLE A fluidized bed reactor consisting of a 1% ID staingaseousreaction mixture of tungsten hexafiuoride and rhenium hexafluoride atthe bottom of the reaction chamber. This phase was contained for 1% to 3hours which produced coating thicknesses of 16 to 48 microns on the lesssteel reaction chamber having a wire mesh bed sup- 5 gi fi' h t te oat dU 1 port at the bottom was used for co-deposition of tungstenwere z ig xi T z fisgf g z a 1d 2 1. 13 rhenium alloys onto U0 particles. Thereaction chamber Phot I0 I f th 2 m 1 was contained within an outer 2nickel pipe which was the z g S i g3 gi g e a in turn disposed within aresistance furnace. Sources of p P W re r v01 S o 1 m Goa hydrogen andargon were connected through a common The rhPmum4ungsten alloy coatmgsappear columnar gas line to the bottom of the nickel pipe for initialpurging when Viewed under polaflzed hght- Chemlcal alum/5159f andfluidizing the U0 charge. Separate rh i h the resultlng coated particleswas effected by cinchonme fluoride and tungsten hexafluoride gas flowswere metered Pfeclpltatloll a lgnltlqn to P f for tungsfen and d passedth h a common fi i t th b tt f h trlphenylarsonlum chlorldeprec1p1tat10n for rhenium and reaction chamber, with the end of thecommon line ter- 15 the coating composition determined for each run. Theminating immediately below the wire mesh screen. The results are givenin the table below.

TABLE Charge Gas Composition Coating Plating Etl. Percent Temp., Weight,Size, H2 Inert VVFs ReFa WFa/ReFe Rate, Re, Thickness, WFo RcFs C. Grams[L ratio ,u/hr w./o.* [L

'Phenium in coating.

common gas line external to the reaction chamber was heated by a heatingcoil to preclude plugging of the line by liquefaction of the rheniumhexafluoride.

While tungsten hexafluoride could be transferred at room temperature,special precautions were taken with the transfer of rhenium hexafiuorideto insure against reaction with moisture and air and liquefaction of thegas during transfer to the reaction chamber. The rhenium hexafluoridegas flow rate was provided by bubbling helium gas through a container ofliquid rhenium hexafluoride which was maintained at a controlledtemperature of about C. The quantity of rhenium hexafluoride transferredby the helium carrier gas into the reaction chamber was thus indirectlycontrolled by directly controlling the flow of helium through the liquidrhenium hexafiuoride.

Deposition temperatures were monitored by a thermocouple which wasdisposed within the reaction chamber submerged in the U0 charge.

To determine the eifect of deposition variables of temperature and gasflow rates, separate runs were made as follows: Various weight chargesof U0 particles (105- 595 microns) were suspended in the reactionchamber for the separate runs. The reactor was then purged with argonflow (12 s.l.p.m.) While bringing the reactor and its charge todeposition temperature, and then hydrogen flow was initiated.

At this point co-deposition of rhenium-tungsten alloy was commenced bythe simultaneous introduction of a At low temperatures such as 390 C.the low plating efiiciency of WF (6.9%) and ReF (42.4%) makes thedeposition process unattractive. At 750 C. in addition to the lowefficiencies, the coating was nonuniform and contained voids. Runs 2-5produced high quality coatings and show the efiect of the WF /ReF ratioat 440-450 C. and at 550 C. on the rhenium content of the coating. At650 C. (Run 6) the decrease in ReF efficiency, at a WF /ReF ratio whichproduced good results at 440- 550 C., renders this temperatureuneconomical.

What is claimed is:

1. Spherical ceramic nuclear fuel reactor particles having a uniformbonded coating of tungsten-rhenium metal alloy.

2. The article of claim 1 wherein said fuel particle comprises 210-500micron U0 and said coating comprises by weight percent 21% rhenium and79% tungsten.

References Cited 1964, Abstract #36,228.

CARL D. QUARFORTH, Primary Examiner.

M. J. SCOLNICK, Assistant Examiner.

